If someone could explain how to use the function, that would be great. I don't understand the parameters.
Thanks
First parameter is a pointer to the chars. c_str() gives you that pointer from a string object. Second parameter is optional. It would contain a pointer to the next char after the numerical value in the string. See http://www.cplusplus.com/reference/clibrary/cstdlib/strtod/ for more infos.
string s;
double d;
d = strtod(s.c_str(), NULL);